The invention relates to a combination drill bit which is designed to drill holes by annular cutting and continuous core breaking.
The new combination drill bit is designed to carry out a process for drilling by annular cutting and continuous core breaking. Experiments were carried out with jet beams cutting a core by annular cutting, which core is broken by a rock bit, cf. Maurer, W. C. Heilhecker, J. K. and Love, W. W., "High Pressure Drilling"--Journal of Petroleum Technology, July 1973. These experiments resulted in an increase of the drilling rate by 2-3 times. The problem in utilizing a jet beam is that it requires a down-hole pump, which is able to produce the very high pressure necessary to enable the liquid beam to cut the formation.
Previously, PDC (polycrystalline diamond compact) cutting elements and rock bits with teeth were combined, but then mainly with the intention to limit drilling advancement in soft formations in order to avoid clogging of the cutting elements, cf. U.S. Pat. No. 4,006,788.
At present, mainly two kinds of drill bits are used, i.e. PDC drill bits and rock bits. PCD drill bits cut the formation with the aid of an edge comprised of a number of PCD cutting elements. Due to the fact that the cutting elements rotate at the same rotational speed about a common axis, cutting speed will vary from zero at the center, to a maximum outermost on the periphery of the drill bit. It is, thus, impossible to achieve an optimal cutting speed of all cutting elements at the same time.
The cuttings formed when PDC cutting elements are used, often are very small, resulting in the fact that very limited geological information can be extracted from them. PDC-bits were constructed which cut a small core for use in geological analysis, cf. U.S. Pat. No. 4,440,247. Drilling operators reported that the their effect as regards acquiring larger pieces is quite low.
The edge of a present PDC cutting element is 90.degree. and sharp. Consequently, it is comparatively weak and tends to chip.
Rock bits break up the formation, by teeth which are mounted on the rock bits being urged towards the formation by so high a force that the formation will break under and around said teeth. Due to the relatively plane face of the hole bottom, crack propagation due to each tooth penetration is of relatively small effect as regards the volume to be drilled. If the volume to be broken is acquired in the shape of an unstabilized core, the efficiency of each tooth penetration will be considerably improved.
Conventionally, the principle of annular cutting with continuous core breaking is not used, at present, for drilling holes. There are a number of patents based on this principle. According to one patent, diamonds baked into a matrix are used. This system provides for more grinding than cutting, requiring high rpm to achieve a satisfactory drilling advancement. The central rolling cones, which are used to break the core, then have to be run at too high rpm, cf. U.S. Pat. No. 3,055,443. According to another patent, edges of tungsten carbide are used, resulting in a very limited life of the drill bit due to insufficient resistance to abrasion of the edges. The last mentioned drill bit does not generate a cavity about the core before it is broken, i.e. the internal wall of the core drill bit has a stabilizing effect on the core, cf. U.S. Pat. No. 3,075,592. A third patent discloses utilizing cutting edges requiring channels/grooves in front of/behind the edges. The channels/grooves must be large enough to permit the pieces of broken core to pass to the outside of the drill bit. The core is broken by the aid of a toothed roller which has too much scraping effect due to its geometry. This will cause the teeth of the roller to be worn down far too rapidly. Nozzles are used to flush the toothed roller and to moisten the core so as to weaken it, cf. U.S. Pat. No. 2,034,073.